photo: iStockPhoto You may find company among zombies and the living dead this Halloween, but the science of the real dead might be just as bloodcurdling.

Pathologists and medical examiners piece together how and when a person likely died. Fleshing out these details clue investigation teams into whether a person succumbed to natural causes, an accident or by the actions of someone else. But to make accurate estimates of time of death, they need to understand the science behind body reactions once the heart stops beating.

Though there are many factors to consider, livor mortis, a phenomenon involving the shift of blood in the body, is often examined alongside other types of clues such as rigor mortis, or muscle stiffness.

Every second, a living person's heart pumps oxygen-rich blood through the entire body in an extensive network of veins, arteries and capillaries. Blood gives organs and muscles the nutrients and warmth they need to work properly.

But what happens when the heart stops pushing blood throughout the body, and why does it matter to scientists?

For starters, the blood stops moving and gravity takes hold. If a person dies and falls on his back, blood will settle in the areas closest to the ground and will slowly drain from the body's front side. Indeed, such movement gives the dead the pale appearance we associate with a lack of life. Blood doesn't disappear, it simply moves to the backside of the body closest to the ground.

As death settles in, the blood begins to congeal, making the back of the person's body purple and pink from the influx of blood. After eight to 12 hours, the blood will usually stay that way.

But the parts of the body in direct contact with objects or the floor will appear pale because of the pressure placed on the skin's capillaries. One medical examiner and pathologist at a lecture on forensics and murder explains that when you press and hold your fingertip on the top of your hand for a few seconds, you'll release it to find it's paler than your surrounding skin. Then, it quickly turns back to your normal skin color. This is because your body recirculates the blood you temporarily pushed away.

The same concept applies when livor mortis sets in, except there's no heartbeat to push blood back into parts of the body.

Forensic scientists find livor mortis somewhat useful in helping determine when a person died. By gently pressing on areas of the deceased's skin that show livor mortis, pathologists and examiners can try to estimate when a person died.

If the area turns pale and then becomes colored again, that means the blood has not fully congealed yet and the person may have passed within the last 12 hours or so. If the area remains the same darker color, it suggests the victim may have been dead for longer than 12 hours.

They can tell if the body had been moved or tampered with since the time of death. For instance, if police find a woman lying face-down, but experts find signs of livor mortis on her backside, it's possible she died on her back and was flipped over more than 12 hours after death.

An example of livor mortis can be found here (Caution: the image contains a deceased body and may be graphic for some).